(Adds statement from UNHCR, paragraphs 7-9)GENEVA, May 13 (Reuters) - Prince Sadruddin Aga Khan, a former head of the United Nations refugee agency UNHCR and U.N. humanitarian envoy to Afghanistan, has died at age 70, his office said on Tuesday. Sadruddin was uncle to the current Aga Khan, Karim, who is spiritual leader of the world's 15 million Ismaili Muslims. A spokeswoman at his Geneva office confirmed he had died, but declined to give further details. Swiss news agency ATS reported Sadruddin died in a Boston hospital in the United States on Monday after a long illness. Sadruddin, an expert in Islamic art, was also head of two Geneva-based environmental groups, the Bellerive Foundation and Alp Action. Harvard-educated, he served as U.N. High Commissioner for Refugees from 1965-77 and was coordinator for U.N. humanitarian and economic assistance in Afghanistan from 1988-90. Named U.N. refugee chief in 1966, at the age of 33, he became the youngest person to hold the post and left an "indelible print" during his 12-year tenure, the U.N. High Commissioner for Refugees (UNHCR) said in a statement. "He was at the helm of the U.N. refugee agency during one of its most difficult periods. This included the 1971 Bangladesh crisis, which uprooted 10 million people, the 1972 exodus of hundreds of thousands of Hutus from Burundi to Tanzania and the Indochinese boat people tragedy of the mid-1970s," it said. The UNHCR also paid tribute to Sadruddin's "key role" in finding new homes for tens of thousands of Asians expelled from Uganda by President Idi Amin. Last year, Newsweek magazine said Sadruddin made an unlikely "fire-breathing ecowarrior" with his "Iranian passport, American education, Geneva address and long career as a U.N. diplomat". Among the awards received during a long international career were Commandeur de la Legion d'Honneur from France and the U.N. Human Rights Award and Dag Hammarskjold Honorary Medal. "He was a man of passion and conviction," commented French-language Swiss Television.